How the Emerald Ash Borer Reached North America

Many of us look back on the travails and the heroic efforts of our ancestors to journey to their new homes, the countries of our birth, with admiration; legitimately so. Nevertheless, ever since people first traveled the ocean waves, they have not journeyed alone. Some have deliberately taken their domesticated animals or plants with them when migrating, but all have given passage to stowaways. These are usually rodents such as mice and rats; invertebrates such as ants and roaches; parasites like tapeworms; bacterial, fungal and viral microbes; and weed plants in the form of seeds stuck to clothes or baggage.

While these organisms can spread across the land without human assistance, across the seas is rarely possible for them. With increased shipping and the relatively recent addition of air flight, alien invasive species and their impacts on virtually defenseless ecosystems has become the most significant biosecurity issue of the modern world. It may be rivaled by the threat of bioterrorism; but that is basically the same thing, done deliberately and with malicious intent.

In the first decade of the 21st century, one of the alien invasive species generating great concern in North America is Agrilus planipennis Fairmaire; which has been given the common name emerald ash borer or emerald ash borer beetle since its arrival. First discovered near Detroit in southeastern Michigan in the summer of 2002, this infestation has steadily spread and is now found in Illinois, Indiana, Maryland, Michigan, Missouri, Ohio, Pennsylvania, Virginia, West Virginia and Wisconsin in the United States and Ontario and Quebec in Canada.

The ash tree species of North America do not have the biochemical protections the ash species of northeastern Asia, where A. planipennis is native, evolved as defenses. Defenseless before this invader, they have died in their tens of millions. Economic costs to forestry industries, landowners, nursery operators and taxpayers have reached tens of millions of dollars so far.

Although discovered in 2002, the beetle probably arrived in its larval form well before this, sometime in the 1990s. It is thought they most likely arrived in cargo shipments from Asia. Ash wood is a commonly used material for dunnage (loose packaging material to support a cargo), particularly in China and northeastern Asia.

While the adult beetle lives on the outside of the trees eating the foliage, they lay their eggs in the crevices of the bark. When the eggs hatch, the larvae burrow through the bark into the sapwood of the tree, where they eat zig-zagging tunnels for three to six months. If recently harvested ash wood was used for packaging a shipment bound for the US, a few of the larvae may have survived a journey to Detroit and matured into adults.

Adult emerald ash borers are capable fliers, among the best of the beetles of the Agrilus genus, able to make a dispersal flight of several kilometers. A dispersal flight is an instinctive behavior made by a new adult upon emerging from the tree. A few survivors of the journey reaching a north American ash tree would have found they had come to their version of paradise. They mate between seven to ten days after emergence and the females lay from 68 to 90 eggs. With no inhibiting biochemicals, larval survival might be close to 100 percent. With no natural predators, parasites or pathogens in their new environment, the population growth rate would be exponential. So that within a few years of arrival, their impact on native North American ash species would have become noticeable, resulting in their discovery in 2002.